U.S. Pat. No. 5,638,223 discloses a projection objective that is assembled from a number of modules or individual subassemblies that are connected to one another and in this way are assembled to form a common objective housing. Each subassembly can in this case have a number of optical elements. The connection of the individual subassemblies to form a unit is performed by a multiplicity of flanges connected to one another via screwed connections.
However, it is disadvantageous of such a projection objective that the adjustment of the individual components to one another and to the objective housing, and their alignment with the optical axis (z-axis) is very complicated. On the occasion of exchanging or repairing individual subassemblies or optical elements, there is a renewed need for the entire objective to be aligned and/or adjusted with a correspondingly high outlay.
A modular design of an objective or lens system is described from “Vukobratovich, Modular Optical Alignment, Proceedings of the Spie, Spie, Bellingham, Va., US, Vol. 3786, Jul. 20, 1999 (Jul. 20, 1999), pages 427-438, XP009006617, ISSN: 0277-786X, page 427-page 428”.
Reference is made to the older German application DE 102 46 898 for the further prior art.
Also known are projection objectives (for example DE 102 25 265.3) as so-called catadioptric objectives with a beam splitter element that reflects the incident linearly polarized light from a vertical into a horizontal part of the objective. After reflection of the light at a reflecting element and a change in the polarization direction by 90°, the light passes through the beam splitter element and is subsequently deflected at a reflecting mirror in such a way that the beam subsequently impinging on a wafer corresponds in direction to the direction of the entrance beam into the objective.
The optical elements used in the objective are generally held in individual mounts and then connected to the objective housing via the latter.
A problem with projection objectives, in particular when the latter are operated in very short wavelength regions such as, for example, 193 or 157 nm, consists in that a very high accuracy is required both in positioning them in the objective housing and when positioning the optical elements with respect to one another, in order to achieve the desired high imaging accuracy. A high imaging accuracy requires, in part, that the optical elements such as, for example, the beam splitter element, be removed from the objective for remachining, and then be remachined, for example have their surface corrected. It is to be ensured here that after reinstallation the same installation conditions obtain again with narrow tolerances.
At the same time, the positionings of optical elements relative to one another, in particular of optical elements that interact directly, are also to be maintained. This holds, in particular, for the beam splitter element and the deflecting mirror arranged downstream thereof in the beam direction. There is, moreover, the requirement that the optical elements be mounted in mounts that have a high natural frequency, but at the same time are intended to decouple the optical element from deformations.
Reference is made to U.S. Pat. No. 6,362,926 and U.S. Pat. No. 6,043,863 for the further prior art.
Concerning high-quality optical imaging devices such as in the case of the projection objective described at the beginning, for example, the optical elements must have very high accuracies with reference to their imaging quality. This holds, in particular, for lenses and mirrors with reference to their surface, for example to their fits. The same holds for coatings for blooming the surface.
Reference is made to U.S. Pat. No. 6,276,398 B1, the older DE 102 59 536.4, US 2002/0171922 A1, WO 02/084671 A1 and U.S. Pat. No. 6,256,147 B1 for the relevant prior art.